The primary goal of the proposed research is to elucidate mechanisms responsible for the coordination of collective cell movements that, in turn, are required for proper tissue assembly and morphogenesis. During vertebrate gastrulation, a dramatic series of cellular rearrangements culminates in the segregation of germ layers and the establishment of the vertebrate body plan. As these events unfold, the cells and tissues involved generate and respond to mechanical forces that are borne by both cadherin and integrin adhesions. Few studies have attempted to link this ebb and flow of mechanical signals to the emergence of specific motile behaviors of individual cells or groups of cells in vivo. Our overall hypothesis is that mechanical forces play an essential instructive role in morphogenesis by organizing and directing the cellular behaviors responsible for bulk tissue movements including the collective migration of cells on extracellular matrix (ECM) substrates. Using the mesendoderm of the amphibian Xenopus laevis as a robust model for collective cell motility, we have discovered a novel mechanically-responsive cadherin complex with links to the keratin intermediate filament (KIF) cytoskeleton. These cadherin-dependent cell-cell adhesions resist and balance integrin-dependent traction forces generated at the front of each cell. The resulting force anisotropy is crucial to the organization of collective cell movements in the mesendoderm. In this project period we will determine the steps involved in directing cell protrusive activity i response to tugging forces on cadherins and establish the contributions of intermediate filaments to this process. There are three specific aims. The first will establish the relationship between cadherin tugging forces and activation of the Rho family GTPase Rac1. We will address the hypothesis that there is an antagonistic relationship between Rac activity and the KIF cytoskeleton that regulates protrusive activity in these cells. The second aim will establish the relative contributions of two types of forces to the progressive closure of the circular mesendoderm tissue: distributed traction stresses generated by individual cells on fibronectin vs. contractile forces generated by the circumferentially arranged cells at the leading-edge of the tissue. The final aim will employ optically based approaches to selectively perturb cadherin linkages to either the actin or keratin filament cytoskeletons. These studies will address the functional importance of the cadherin-KIF complex to mechanosensing, regulation of polarized protrusive activity, crosstalk with integrin-ECM adhesions, and the coordination of collective cell migration. This research has broad relevance to fundamental problems of tissue regeneration and repair, normal tissue morphogenesis and its dysregulation in diseases, which include cancer and metastasis.